(1) Field of the Invention
The present invention relates to a crosslinked polyester for use in the production of a toner. More particularly, the present invention relates to a cross-linked polyester resin which has good and balanced low-temperature fixing property, offset resistance, blocking resistance and abrasion resistance, which is used for developing a latent image formed by the electrostatic printing method or the like.
(2) Description of the Related Art
In the conventional electrophotographic process, in general, a latent image is formed on a latent image-forming roller composed of a photoconductive photosensitive material by charging and light exposure, the latent image is developed with a toner, the obtained toner image is transferred onto a paper sheet or the like, and the toner image is fixed by heating or application of a pressure to form a print having a fixed image.
It is desired to perform this image-forming treatment through a latent image at a high speed, and a hot roller fixing method capable of performing the fixing operation at a high thermal efficiency and an elevated speed is widely adopted at the fixing step.
Recently, the desire to further elevate the speed is increasing, and for satisfying this desire, the toner must have a good low-temperature fixing property to fix a toner image at a high speed. For this purpose, the softening temperature of the binder which is the main component of the toner must be lowered and the adhesiveness of the binder must be improved. However, if the softening temperature of a resin is lowered, the glass transition temperature (hereinafter referred to as "Tg") is simultaneously lowered, and therefore, a good storage stability cannot be obtained and blocking readily occurs. Moreover, the toner powder supplied by a high-speed feed machine undergoes a strong external force by violent agitation or the like, and hence, the toner powder is excessively finely divided and the balance among entire physical properties is lost.
Extensive researches were made for solving these problems and several proposals were made.
According to the first proposal, an offset-resistant toner is provided by using, as a binder, a crosslinked polyester obtained by reacting an etherified bisphenol. A component with a dicarboxylic acid component to obtain a linear polyester and adding a trivalent or higher polyvalent monomer component, for example, trimellitic acid, to form a crosslinked structure.
The second proposal relates to a process for improving the low-temperature fixing property by reducing the molecular weight of a binder to improve the flowability of the binder.
According to the third proposal, the low-temperature fixing property is improved by using a binder having long-chain aliphatic hydrocarbon units.
The first proposal, however, has a problem in that the fixing property is not satisfactorily good, and when the fixing operation is continuously carried out at such a high speed as at least 50 prints per minute, the toner is not sufficiently heated at the fixing temperature adopted and the fixing strength is often unsatisfactory. Moreover, the first proposal involves a problem in that fine particles are accumulated as the operation is continued, and the image quality is degraded.
It was found that in the process of reducing the molecular weight according to the second proposal, the offset resistance is degraded and no practical toner can be prepared.
The glass transition temperature of the polyester of the third proposal is too low, and the storage stability is degraded.